System for reproduction of sound by means of light-sensitive cells



Oct. 24, 1933. A. POULSEN ET'AL 1,931,829

SYSTEM FOR REPRODUCTION OF SOUND BY MEANS OF LIGHT SENSITIVE CELLS Filed Oct. 22, 1929 Ga 9x2] [427*] 6209*? fez M Patented Oct. 24, 1933 M lambs-w U'NiT E sTArrs PATENT orsrilcs SYSTEM FOR REPRODUCTION OF SOUND, BY MEANS ':0F LIGHT-SENSITIVE CELLS ,Arnold Poulsen, Hellerup, and Axel Carl Georg 7 Peterson, Copenhagen, Denmark Application October 22 1929, SeriallNo. 401,581,

and .in Norway October 31, .1928

2 Claims. (Cl. 250-415) In the reproduction :of sound recorded 011 .2, light-sensitive membrane, for instance :a ifilm, it .is known to .use;a;light-sensiti.ve cell, for instance a selenium cell :or a photo-electric cell, which is i5 exposed taught-passing through thesoundrecord formed on the Efilm, and is inserted in .an electric .circuitcontainingan impedance. 'In consequence :of the current variations produced in this circuit by the varying lighting :of the light-sensitive cell I the difference of potential which exists between the two endsof the impedance, and is impressed onto the grid of 1a thermionic-valve amplifier,

will vary according to the lighting .of the lightsensitive cell. Ordinarily the light-sensitive cell 115 is inserted directly in the grid circuit-of the amplifier,1and the said impedance is inserted :between -:the two wires leading from .the cell to the amplifier; and in parallel with .the latter. 7

1 The insertion of .a light-sensitive cell :zdirectly in the grid circuit of an amplifier valve, or rather sin direct conductive connection with such agrid circuit, involves 'theiconsiderable drawback that the .connecting wires between'thecell and the amplifier must be'relatively short, because otherwise the mutual capacitybetween these connecting wires, owing to the high resistance of the light-sensitive-cell as well as. the considerable loading impedances occurring in the circuit, 'will .have a considerable weakening 'effectfon the transformation of the variationsoi :current into variations of potential inxthe above mentioned impedance, especially in the case :of current ya- ;riations with highfrequency, :so that-a distortion will be produced which cannot subsequently be v compensated. .A further consequence of the said considerable impedances is that very great-disturbances will be induced in the said connecting wires, if they are long and come within the range When during the 1 reproduction the sound film is moved synchroof external disturbing fields.

nously with a picture film such fields will occur in the neighborhood of the picture mechanism owing to the driving motorv or arc of the same.

It is well known that thefldifference between therpotentials induced by a disturbing'conductor in two conductors parallel thereto depends essentially on the difference between the capacities toearthof the 'two conductors and on the magni-' tude of the impedance occurring between the conductors, and increases with the said difference and the said impedance. Now in systems for refproduction of sound by means of light-sensitive cellsthe two wires leading from the cell to the amplifier device will necessarily have highly dif- .55 -ferent capacities to earth, because one of the wires will always be connected to the battery inserted .in the circuit of the cell, whichbatteryhas a highcapacity to earth, whiletheother wire will have a relatively low self-capacity toearth. As the impedance inserted between the wires in the system is always relatively large,;.thedisturbance caused in the system by stray; fields may consequently becomeoquiteconsiderable. 1

In order to avoid the above mentioneddrawbacks, especially when photo electric cells are used, which cells of course havea very-high resistance, it has been proposed to omit the con necting wires between the cell and the amplifier entirely by building the light-sensitive cell together with the first stage or stages-oithe amplifier device. The light-sensitive cell must necessarily be placed in the recording apparatus proper in which the film-feeding mechanism is :disposed,

and this construction consequently involves the drawback that vibrations produced by the filmfeeding mechanism are propagated to the amplifier, for which reason the amplifier has to be suspended resiliently, and theamplifier valves have to be supported resiliently and, furthermore, (that ,it becomes necessary to protect not only the cell proper but'also the amplifier againstdirectactuation irom external stray fields. I When using selenium cells the resistance .of

which is not so high as the resistance of; photo- .placed at a point where .it will not be, exposed ,to the influence of stray fields originating from the recording apparatus .or the motors, arcs etc. required for operating the same, and tha't'the induction produced in the connecting wires owing to the-influence of. external stray fields will be reduced to. a minimum. v

The desired object is attained accordingtb the invention by inserting the light-sensitivel'cell in the primary circuit of a transformer serving to transform downwardvery considerably the variations of potential produced in the circuit of the selenium -.cell, whereafter :the thus reduced variations of potential are transmitted by way of '.r is the grid leak resistance.

long wires to the primary winding of another transformer in which the variations of potential are transformed up to their original or possibly a higher value, and are impressed on the grid of a thermionic-valve amplifier.

The drawing shows diagrammatically two different constructions of the invention,

Fig. 1 showing the wiring diagram of a lightsensitive cell and a thermionic-valve amplifier connected thereto by way of transformers,

Figs. 2 and 3 two different constructions of corresponding wiring diagrams for a reproducing system in which a selenium cell is used as lightsensitive cell, and V Fig. 4 a reduced diagram of the wiring arrangements shown in Figs. 2 and 3.

In the construction shown in Fig. 1 c is a light-sensitive cell, for instance a photo-electric cell or any other suitable cell, inserted in a circuit containing an inductance L1 and a battery B. One terminal of the light-sensitive cell c and one terminal of the battery B are connected to earth as shown, so that the circuit of the lightsensitive cell is closed through the earth.

The impedance L1 forms the primary winding of a step down transformer O the secondary winding m of which is connected to the primary winding 'n of step-up transformer O by means of two connecting wires a and b of any suitable length. One of these wires, viz. the wire I), belongs also to the circuit of the light-sensitive cell.

The secondary winding L2 of the transformer O is inserted in the grid circuit of a thermionicvalve amplifier T of known nature. It is the blocking condenser in the said grid circuit, and As shown by Fig. l the wire I) is connected to one end of each of the four transformer windings L1, m, n and L2.

The alternatingcurrent generated in the circuit of the light-sensitive cell 0, when the latter is irradiated, causes corresponding variations of potential across the terminals of the primary winding L1 which variations of potential are transformed down by the step-down transformer O, the ratio of transformation n being suitably chosen very high, and in the transformer 0' the variations of potential are transformed up to their original value, the ratio of transformation being consequently and are then transmitted in known manner to the amplifier T. It will be seen directly that the impedance between the long wires 11 and b is very low in comparison with what it would be 1 if thelight-sensitive cell 0 were connected directly to the grid circuit in the heretofore com monly known manner, whereby for instance a large resistance is inserted between wires corresponding to wires 11 and b of applicants device. In consequence of this small difference of impedance between the wires a and b in the constructions shown in Figs. 1 to 3 the difierence between the potentials induced in the said two wires by disturbing external stray fields, and consequently the disturbances caused thereby will also be very small. Then the influence of the capacity between the two long wires a and b will be reduced in the ratio n and will therefore be practically annihilated when a suitable ratio of transformation n has been chosen. In order further to reduce the inductions caused by the stray fields the battery B may be placed as shown in Fig. 1, in connection with the part of the recording system containing the amplifier T, and further the transformer O and the lightsensitive cell may be encased in a housing H of highly conductive material, for instance copper or iron, so that the eddy currents induced in the casing H will protect the cell c and the transformer 0 against the influence of external fields.

The casing H may consist of two shells one of which encloses the other one, and one of which is made for instance of copper, while the other one is of magnetic material for instance iron. The battery B may form part of the anode battery of the amplifier T.

Fig. 2 shows one construction of a recording system built according to the invention and using a selenium cell as the light-sensitive cell. The selenium cell S is here coupled to the amplifier valve T in a manner exactly similar to that of the cell c in Fig. l, but an ohmic resistance r is inserted in known manner in series with the battery B and the cell S as well as the impedance L1 the said resistance r being similarly in known manner adjusted so that in connection with the inductance of the transformer winding L1 it will serve partly to compensate any distortions which might have been caused in the sound record by the amplitudes of sounds of low frequencies being suppressed during the recording process, cf. the specification of British Patent No. 248,148, and partly to compensate in similarly known manner, the distortions due to the selenium cell S, cf. the specification of Danish Patent No. 34,901 of August 12, l925..

Fig. 4 shows a reduced diagram of the wiring arrangements shown in Figs. 2 and '3, the said diagram serving to illustrate graphically how the impedances of the windings L1 and L2 and the resistance r are included in thecircuit of the selenium cell. i

In the recording system shown in Fig. 2 the impedance of the winding L1 has to be chosen so small that the product w.- L1, for the highest occuring value of the frequence 40 will be small relatively to the resistance or" the selenium cell, while on the other hand, if forinstance a photoelectric cell is used in the system shown in Fig. '1, theimpedance of the winding L1 must be chosen so high that for all frequencies the product L0. L1 will be great relatively to the resistance of the photo-electric cell, or since this is practically impossible a suitable resistancemust be inserted in parallel with the impedance L1 or L2, all for the purpose of avoiding unintentional distortions.

Having now particularly described and ascertained the nature of our said invention and in what mamier the same is to be performed, we declare that what we claim is:-

1. A system for reproducing sounds from phonograms'comprlsing a light-sensitive cell adapted to be illuminated by light passing through the phonogram, a source of current, a step-down transformer, a current circuit including in series the primary winding of the step-down transformer, the current source and thesaid cell; a step-up transformer, arranged together with the said current source at a relatively great distance from the light-sensitive cell and the step-down transformer, suitably long conductors by way of which the primary winding of the step-up transformer is connected across the secondary winding of the step-down transformer and a thermionic tube having a grid circuit in which the secondary winding of the step-up transformer is inserted.

2. A system for reproducing sounds from phonotransformer, a housing of conductive material ena casing the light-sensitive cell and. the step-down transformer, suitably long conductors by way of which the primary winding of the step-up transformer is connected across the secondary winding of the step-down transformer and a thermionic tube having a grid circuit in which the secondary winding of the step-up transformer is inserted.

ARNOLD PoULsEN. AXEL CARL GEORG PETERSEN. 

